That's a lottery. It's very hard to predict how CPU requirements will change in the future regarding multi-threaded vs. single thread performance. It's not even that easy to guess what your usecase will be a few years from now.
Usually it's not worth it. By the time you need a 5950X, you might even be able to buy a faster cpu for the price difference you save now
Just in case it's not clear or obvious... the UPS rating in VA is NOT the same thing as the maximum power it can supply.
A 1000 VA UPS can not output 1000 watts continuously, or is not supposed to - the internals may be oversized, or if you're lucky they use the same components as bigger models and just label it with a lower rating.
That's why you'll see some UPS models say stuff like 1500 VA true sine power output max 900 watts or something like that.
You're looking at peaks of 320 watts for the 3080 without overclocking, let's say it peaks at 350 watts when overclocked.
A processor like the one mentioned will peak at around 150-200 watts, if you overclock it heavily it may go to around 250 watts.
You're looking at around 20 watts for the fans and water cooling and maybe 20-40 watts in 3.3v and 5v for everything else in your computer (rgb strips, onboard audio, lan, ssds, keyboard, mouse, other usb devices etc)
So let's say the components pull peaks of 650-700w. A platinum efficiency psu will produce these 700 watts with around 92-94% efficiency... let's be conservative and say 92%... that means the 700w were 92% of what pulled from mains... so the psu actually pulls around 750-760 watts and around 50-60 watts are lost as heat in the psu.
When power is lost, the UPS has to take 10.8...13.8v from lead acid battery and uses an inverter to produce 110v AC or 230v AC... that's done with around 80-90% efficiency. So that 750w becomes around 800-850 watts of DC power pulled from battery.
So you can easily do the math ... 850 watts / 10.8..13.8v = 62..78 A of current pulled from a 12v lead acid battery... the current increases from that 60-ish Amps and goes up as the battery voltage goes down.
The cheap 500-750 VA UPSes use a single 7-9Ah lead acid battery... let's say a 1500 VA UPS uses a bigger 20Ah battery, here's a standard UPS battery : https://www.digikey.com/en/products/detail/zeus-battery-products/PC20-12NB/12570398
So with such 20Ah battery, pulling 800w or so, you'll get maybe 10 minutes of running on UPS ( looking at around 50A because I assume as soon as the power goes off, you'll minimize the game or have Windows "power management" to throttle the cpu and reduce power consumption ... so you'll have that peak current when gaming for let's say half a minute to 1 minute until you realize the power failed and pause game and then the power consumption should drop to whatever's idle, average power consumption of 200-300w
The above battery charges at up to 6A but it's not a good idea to use such high current, a ups most likely charges at around 2-3A ... you'd be looking at around 8..10h to charge this battery when fully discharged. Says at the top in datasheet.
So the reason I suggested an UPS that uses two batteries, is because most likely such UPS will have two 9Ah..12Ah standard mass produced batteries, easy to replace when dying, and if the power fails when you're gaming and your PC pulls a lot, the two batteries share the load, so each battery sees half the current, or only around 20-30A instead of one battery being abused with a sudden 800w pull (around 60A)
Even better choice would be one of those rackable UPSes which use up to 4 lead acid batteries as they're designed to work either from datacenter 48v DC or from mains.
You can find refurbished / used ones with or without batteries for cheap, like 100-300$
Here's an example : https://www.apc.com/shop/my/en/products/APC-Smart-UPS-C-1500VA-LCD-RM-2U-230V/P-SMC1500I-2U
$139 without batteries : https://www.ebay.com/itm/APC-UPS-SMC1500-2U-1500VA-LCD-Smart-Uninterruptible-Power-Supply-No-Batteries/254755515197
$280 WITH new batteries (184 sold, 8 still available, looks like someone cleared a datacenter or something) : https://www.ebay.com/itm/APC-SMT1500RM2U-Smart-UPS-1500VA-RM-2U-LCD-New-Batteries-1Yr-Wrnty-FreeShip/113361095725
$299 WITH new batteries (130+ sold, >10 available) : https://www.ebay.com/itm/APC-SMT1500RM2U-Smart-UPS-Power-Backup-LCD-1500VA-1000W-120V-Rackmount-New-Batt/223987326330
They did, but not close to a clear answer they did not say if it was with fans or with a radiator or with nothing at all and by the size of the gpu im getting (strix 3080) it would not fit if the total clearence was with no fans installed as of now it will fit but the radiator will have about 10 mm of space between the gpu and rad+fans
I had a long chat with corsair support until I got a clear answer but here is an update to whoever finds this on google the case clearence was measured with the fans on the case (25mm) so the total gpu clearence with no fans is 385mm hope other people find this helpful too